By cellular network is meant mobile telephone networks such as those using the GSM (Global System for Mobile Communications), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunications System), LTE (Long Term Evolution) standards. Typically, a cellular network is divided spatially into a multitude of adjoining radio cells in which the same frequencies are used for telephoning.
Systems for transmitting an alert in the absence of cellular network are used to warn the rescue services in case of accident. There exists a large number of alert transmission systems used, in particular, within the framework of outdoor activity such as hiking, cycling, windsurfing or the like. Indeed such activities frequently take place at sites not covered by a cellular network.
Known systems comprise:                at least one walkie-talkie able to be carried directly by a human being, this walkie-talkie comprising:                    a radioelectric transmitter-receiver able to listen directly to at least one channel used to exchange audio signals alternately with other walkie-talkies, and            a loudspeaker able to transform the audio signal received on this channel into a voice message directly perceivable to a human being who is carrying this walkie-talkie,                        an alert beacon that can be carried directly by a human being, different from the one carrying the walkie-talkie, during an outdoor activity, this beacon comprising:                    a geo-location module able to generate coordinates, in the form of a succession of numerical symbols, corresponding to the current position of the beacon on the terrestrial surface with a resolution of less than 500 m, and            a transmit circuit able to produce an alert signal containing the coordinates generated and to transmit this alert signal produced in the form of an audio signal on the same channel as that used by the walkie-talkie.                        
Such systems are, for example, used in the “Rino 110®” walkie-talkies from the company Garmin®.
This system presents several drawbacks however. For example, in this system the alert beacon is a walkie-talkie identical to the others. It therefore comprises a receiver and a screen for displaying GPS coordinates. Because of these elements, it is bulky and may turn out to be rather impractical to carry during an outdoor activity. Moreover, in this known system, the GPS coordinates can only be played back to a person in a form that is directly comprehensible to this person if the latter is also equipped with a “Rino 110®” walkie-talkie. Conversely, other walkie-talkies of another brand cannot play back the alert signal transmitted by such a “Rino 110” walkie-talkie in a manner that is directly comprehensible to a human being. Thus, if the people in proximity to the accident are equipped only with a walkie-talkie of this other brand, they cannot be warned of the accident and ascertain its location. Chances of rescuing the victim of the accident are thus lost.
Prior art is also known from US2005/258958A1, US2008/316040A1, WO2009/136259A2 and WO2011/060388A1.